How Green Is the Milky Way?

The Milky Way is thought to be entering middle age, running out of the cold gas that is essential for star formation.

By Stephen Luntz

The Milky Way has probably entered its middle age, a transition phase known as the “green valley” between being a hot blue galaxy bursting with star formation and what Swinburne astronomers call a “red and dead” old age.

Mr Simon Mutch and Dr Darren Croton have attempted to find the overall colour of the Milky Way and our nearest large neighbour, Andromeda. The dominant colour of thousands of other galaxies has been plotted, but Croton says that “determining the state of our own galaxy while we’re stuck inside it is very difficult to do. The phrase ‘It’s hard to see the forest for the trees’ really rings true here.

“The properties we’ve looked at all point to the Milky Way having global colours that we’d classify as green – they’re midway along an evolutionary track between ‘blue and star-forming’ and ‘red and dead’,” Croton says, although Mutch admits that the error bars for the team’s estimate of the Milky Way’s colour are quite large.

While measuring Andromeda’s colour is easier, it hasn’t been done for a long time. Mutch says this is because astronomers have preferred to look at its features in detail rather than stepping back to see the bigger picture. However, in Astrophysical Journal the team expresses a greater degree of confidence that it is also in the green valley.

Mutch says it is not known why some galaxies of similar age and size are at very different life stages, but he thinks it has something to do with the activity of their central supermassive black hole. “We have a very exciting opportunity to understand what is causing this,” he says. “New surveys of stellar populations are coming out soon studying how quickly star formation is declining.”

Most galaxies are either distinctively blue or red, with only a minority in between. The team estimates that the typical time for a galaxy to pass through the green phase is around two billion years, which is short in galactic terms.

However, the transition is not smooth. When a galaxy swallows a small satellite galaxy, the arrival of clouds of gas can set off a burst of star formation. If the galaxies are similar in size this can use up all their gas, hastening the transition to red.

The Large and Small Magellanic Clouds are unusually large satellites for a galaxy of our size, so their anticipated incorporation (AS, July 2008, p.43) should give our galaxy a new lease of life. “Those concerned about the Milky Way’s decline needn’t worry just yet,” Mutch says. “Retirement is still a long way off.”

Moreover, a collision between the Milky Way and Andromeda is inevitable (AS, Nov/Dec 2009, p.40), and this should induce a brief burst of star formation as the tiny amounts of gas that each will have left will allow a brief burst of stellar formation, like ageing rock stars making a comeback tour.